Plasterboard or panelling is well-known and traditionally realised from a body or core of gypsum plaster which is deposited, in general by pouring, between two cardboard paper supports ensuring both the mechanical maintenance or framework of the plaster as well as its exterior facing on its outer surfaces.
The fabrication of plaster panelling of this sort is well-known and in particular described in European Patent No. 0 521 804 commonly owned by the applicant of the present invention. In this European patent, provision is also made to deposit, on the exterior surface of the facing paper, a coating slip with the purpose in particular of ensuring a proper aesthetic appearance of the outer layer of the facing paper by avoiding in particular its yellowing, and by ensuring for the facing paper a proper homogeneity of white colour without for all that negatively affecting the manufacturing of the paper, its performance over time and its strength qualities, in particular mechanical.
Nevertheless, a search is constantly being made to obtain plasterboard possessing facing papers presenting good aesthetic properties of their outer layers while being of low cost.
One of the means contemplated consists in realising the facing paper by means of materials presenting a low cost and therefore of quality inferior to the materials, in particular the fibres, used previously. Now, recourse to fibres of reduced quality, in general fibres obtained from recycled paper, has the disadvantage of leading to a facing paper that is darker and less homogeneous in its mass and in its overall hue. The reduction in cost of the base paper, if it actually leads to an appreciable reduction in the cost of the plasterboard, nevertheless has the negative consequence of leading to the production of a paper presenting an external visual appearance that is darker and less homogeneous. On the whole the plaster panelling obtained thereby presents diminished aesthetic qualities.
Restoration of whiteness to paper that is of lower quality has also been contemplated, by increasing the proportion of whitening agent in the coating slip for the paper. Such an inclusion, in addition to the fact that its economic significance is debatable, is not directly envisioned because increasing the proportion of whitening agent in the coating slip for the paper is accompanied by a reduction in the porosity of the coated paper, which has the tendency to reduce the evaporation of water during the setting of the plaster. Consequently, this increases drying time, which reduces the productivity of manufacturing and therefore renders this solution unacceptable.
In other respects, the reduction in the drying properties of the paper, i.e., the increase in the value of Gurley porosity (NF ISO 5636-5) of the paper, is also expressed by risks of detachment of the paper when the plasterboard is manufactured or by the appearance of blisters.